1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gas and manufacturing method thereof.
2. Description of Related Art
Conventional three-way catalysts are not always sufficient to lower a level of NOx in the exhaust and thereby to purify the exhaust gas because of high oxygen concentration of the exhaust gas while an engine operates at lean-burn conditions.
Japanese Unexamined Patent Publication No. 9-38493 discloses a catalyst effective to purify NOx in a high oxygen concentration exhaust gas which comprises a catalytic noble metal and a NOx adsorbent carried in order on a support material of a composite oxide made of alumina combined with lanthanum (La) and/or cobalt (Co). It is said in the publication that while, at high temperatures, alumina reacts with the NOx adsorbent to cause a reduction in its own specific surface area and clogging of its pores, and further to cause sintering of the catalytic noble metal, the composite oxide is prevented from thermal deterioration.
In a manufacturing method of the catalyst, after forming a layer of cobalt alumina complex oxide by impregnating cobalt (Co) into an alumina layer carried on a support and then drying and calcining it, the cobalt-alumina complex oxide layer is impregnated with platinum (Pt) and dried, and is subsequently impregnated with rhodium (Rh), then dried and calcined. Finally, the catalyst is completed by impregnating barium (Ba) into the cobalt-alumina composite oxide layer then drying and calcining it. In the catalyst made by the above method, platinum (Pt) and rhodium (Rh) carried on a single particle of cobalt-alumina composite oxide stay close to or overlap each other.
The catalyst in which two different kinds of noble metal particles are carried on a single particle of support material tends to make an alloy thereof when it is exposed to a high temperature gas, which leads to aggravation of catalytic performance.